Subject: Degradation of GRIM II image quality across the field

From: Alan Watson

Submitted: Sat, 8 Feb 1997 15:19:10 -0700

In our runs since the realuminization, Tom Harrison and I have noticed
variations in focus across the GRIM II field of view. To see what I
mean, look at
ftp://oldp.nmsu.edu/pub/alan/apo/970208/bad.fits
ftp://oldp.nmsu.edu/pub/alan/apo/970208/good.fits
These are stacks of 8x60 second object/sky exposures. A good stretch
is -20 to 100. Ignore the flat-topped saturated stars. In the good
frame, the FWHM is constant at about 2 pixels across the field. In the
bad frame, the FWHM varies from about 2.5 pixels in the upper right to
5 pixels in the lower left, where stellar images ARE DOUGHNUTS. One
pixel is about 0.5 arcsec in these images.
The image quality, when it varies, always seems to be in the sense of
the lower left is poorer than the upper right. However, we've only
really been looking at objects in a fairly restricted part of the sky.
Our hypothesis was that the telescope and the re-imaging optics in the
instrument were being brought out of collimation as the telescope
moved and the instrument rotated. To that end, Mike Ledlow and I took
images of M5 at 0, 60, 120, 180, 240, 300 degrees of rotation during
the engineering run last week. This effect is present in many of the
images, but the seeing is sufficiently bad that it is is not very
obvious and it's difficult to tell what is going on.
At this point there is not much more I can do, but this problem
remains very serious.
Alan Watson
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